1. Field of the Invention
The present invention relates to a zoom lens suitable for an image pickup apparatus such as a still camera, a video camera, a broadcast camera, or a digital still camera.
2. Description of the Related Art
Recently, an image pickup apparatus (camera) such as a video camera or a digital still camera using a solid-state image pickup device has achieved a higher function and overall miniaturization.
An image taking optical system used for such an image pickup apparatus is required to be a zoom lens short in total length, compact, and high in zoom ratio, and have high optical performance over an entire zoom range.
There is known a four-unit zoom lens which includes four lens units, i.e., first to fourth lens units having positive, negative, positive, and positive refractive powers in order of an object side to an image side.
A zoom lens disclosed in U.S. Pat. No. 6,975,461 performs zooming by moving the first to third lens units, and focusing by moving the fourth lens unit.
Each of zoom lenses disclosed in U.S. Pat. No. 7,193,790 and Japanese Patent Application Laid-Open No. 2006-106111 performs zooming by moving all the first to fourth lens units.
The zoom lens of U.S. Pat. No. 7,193,790 includes an F-number determining member positioned between a vertex of a lens surface of the third lens unit, which is nearest to an object side, and an intersecting point of the object-side lens surface and a peripheral portion of the lens. Thus, the entire system is miniaturized by shortening an interval between the second and third lens units on a telephoto end and setting a zoom ratio to five times or more.
Meanwhile, there is known a zoom lens which has an image-stabilizing function for correcting an image blur caused by vibration of the image pickup apparatus. Japanese Patent Application Laid-Open No. H07-128619 discloses a zoom lens which includes first to fourth lens units having positive, negative, positive, and positive refractive powers in order of an object side to an image side, and corrects an image blur by moving the third lens in a direction perpendicular to an optical axis.
Generally, in order to miniaturize the zoom lens, a moving amount of each lens unit accompanying zooming may be reduced while increasing a refractive power of each lens unit forming the zoom lens, and the number of lenses is reduced for each lens unit.
In such a zoom lens, however, aberration variation accompanying zooming increases, resulting in great difficulty in obtaining high optical performance over the entire zoom range or on the entire screen.
In order to achieve a high zoom ratio and miniaturize the entire lens system, therefore, appropriate setting of a zoom type, a refractive power of each lens unit, moving conditions accompanying zooming of each lens unit, a position of an F-number stop for limiting an open F-number beam, and the like is important.
For example, in the case of the 4-unit zoom lens, appropriate setting of a refractive power or a moving amount of the third lens unit which moves during zooming, and a total lens length of the entire system is important.
Inappropriate setting of such structures results in difficulty in obtaining a compact zoom lens which exhibits high optical performance while securing a high zoom ratio.